The present invention relates to a method and apparatus for washing developed photographic materials, and more particularly to a method and apparatus for processing spent wash water for reuse in the washing step.
During the silver halide photography developing process, the photographic material is passed through a fixing bath which makes permanent the development of the film, i.e., fixes the film by removing silver halide from the undeveloped areas. Generally, the fix bath comprises a solution of thiosulfate salts such as sodium, potassium, or ammonium thiosulfate, or combinations thereof, commonly known as hypo. The fixing step is ordinarily the final step before the film is fully developed, but it also usually undergoes further rinsing and drying steps to remove any developing chemicals remaining on the film, including the chemicals from the fix bath.
Incomplete washing results in retention of thiosulfates and argentothiosulfates which can have adverse effects on the photographic image. Reduction of silver complexes, such as Ag(S.sub.2).sub.3.sup.-1 and Ag(S.sub.3 O.sub.2).sub.2.sup.-3, to metallic silver can result in a stain that is most obvious in nonimage areas. Presence of thiosulfates can result in sulfiding of the image silver, causing discoloration. Under conditions of low pH, thiosulfate enters into a reaction with finely divided metallic silver in the presence of oxidizing agents such as dissolved oxygen, oxidized developing agents, or even decomposition products of thiosulfate including polythionates and sulfur. Thus, the washing step removes salt crystals which cause streaks on the film, and the drying removes the water which would cause spots. Generally in the past, the spent wash water containing the rinsed-off chemicals was passed to the sewers in an untreated state, while the spent drying air was passed to the atmosphere.
These traditional photographic washing and drying processes have been criticized recently because unlimited fresh water for washing or rinsing the film is no longer available. This is due in part to polluted streams and population demands on potable water. Further, unrestrained discharge of spent wash water and drying air into the sewer systems, and atmosphere can no longer be tolerated. The chemicals consume oxygen in the discharge streams as the chemicals break down or are converted to more basic or stable forms. For example, thiosulfate salts from the fix solution will consume oxygen in a stream of water into which it is discharged to achieve the more stable sulfate state. This consumption of oxygen is a further pollution of the streams. It reduces the amount of dissolved oxygen otherwise available to support marine life. This consumption or reduction of the dissolved oxygen in the streams is referred to as biological or biochemical oxygen demand (BOD) and chemical oxygen demand (COD).
Another problem with the traditional photographic washing and drying processes has been their inability or difficulty in adapting to mobile film developing situations. When film must be developed in mobile film centers, such as aboard ships or airplanes or in isolated Army field units, there are limitations on the source of supply and quantity of good quality water. Further, the disposal of large quantities of spent wash water may also prove to be difficult and/or a pollution of the streams supplying the wash water.
No photographic washing process has been found which is both nonpulluting and water conserving. Technical Report AFFDL-TR-69-10, February, 1969, entitled "Methods of Conserving and Reclaiming Photographic Wash Water for Reuse", by J. Brennan Gisclard, prepared for the Air Force Flight Dynamics Laboratory, Air Force Systems Command, Wright-Patterson Air Force Base, Dayton, Ohio, discusses the desirability and need for conserving water by recycling photographic wash water. But, the Gisclard report is only a study of ways of conserving water by recycling the photographic wash water, and the effects of the recycled wash water on the film. It does not deal with, nor does it teach, reducing pollutants in the discharge streams, and further only "commercial" quality film is achieved. The report concludes that wash water may be recycled as much as 10 times, with the presence and build-up of salts in the recycled wash water not necessarily adversely affecting the film. The Gisclard report also discloses that an additional rinse may be necessary to remove loose residual salts from the film surface, apparently because the wash water contains a high thiosulfate salt buildup. Thus, while Gisclard teaches water conservation, the recycled wash water when discharged would still contain polluting thiosulfate salts. In fact, it would appear to be even more polluting because of the buildup from recycling.
U.S. Pat. No. 3,531,284 also deals with the problem of reducing clean water consumption, but without effecting reduction of photographic pollutants. A method and apparatus for washing photographic material are disclosed which use a closed loop water reclamation system in combination with an ion exchange technique for washing the film. The reclamation system is actually a heat pump or distillation apparatus which reduces the concentration and contamination level of the fixing chemicals, including thiosulfate salts, in the wash water. While water is conserved, the process and apparatus of U.S. Pat. No. 3,531,284 requires a cleaning step to remove scale buildup, including thiosulfates salts removed from the wash water, and the discharge of that scale would pollute. Further, additional energy is consumed in the process because large amounts of energy are required to run the evaporators and complementary refrigeration equipment.
U.S. Pat. No. 3,186,942 teaches the elimination of sulphides in effluents in the spent alkaline wash liquors obtained in petroleum refineries. The sulfides are treated by an oxidation process using copper or iron group catalysts to convert the sulfides to thiosulfates and then to sulfates. In this way, the oxygen consumption of the effluents in the stream is reduced because the sulfate is a more stable salt. But, this process is not disclosed for use in a photographic developing process, and it does not deal with the water conservation problem associated with the rinsing or washing step in such processes.
Thus, a need exists for a method of washing developed photographic material which reduces the amount of wash water necessary to wash the film, which minimizes the amount of spent wash water discharged, and which minimizes the amount of polluting chemicals in the discharge stream.